There are intriguing reasons to think marijuana improves night vision

A variety of medicinal marijuana buds in jars are pictured at Los Angeles Patients & Caregivers Group dispensary in West Hollywood
Thomson Reuters

For years now, researchers have noticed a remarkable side effect in certain cannabis users. Some people who regularly consume marijuana say that it helps them see in the dark.

Jamaican fishermen have long claimed that smoking marijuana or drinking a cannabis-infused white rum improves their vision, according to a letter published in the journal Nature by M.E. West in 1991.

"I decided to test this belief, and went with a crew on a dark night to a fishing cay approximately 40 miles south of Kingston. The approaches to the cay are shallow with an abundance of coral reefs, only a narrow entrance of deep water allowing boats to get close for mooring," writes West. He waited in the dark for the boat to crash, only to be told that they had successfully docked.

"At daybreak it was impossible to believe that anyone could navigate a boat without compass and without light in such treacherous surroundings. I was then convinced that the men who had taken the rum-extract of cannabis had far better night vision than I had, and that a subjective effect was not responsible."

West used follow-up research to develop a cannabis-based treatment for glaucoma. Researchers are still divided about how effective cannabis is for treating glaucoma, but West wasn't the only researcher to observe these strange effects.

Across the Atlantic, several researchers had noticed Moroccan fishermen and mountain dwellers making the same claim.

In a study published in 2004, Russo et al. note that previous researchers had written of fishermen there, "they attribute their ability to see [in the dark] to the consumption of kif," a local word for a form of cannabis. Russo and co-authors tested the effects of both smoking cannabis and consuming synthetic THC on a few subjects and found a significant improvement in night vision.

Why it works

While there's not a ton of data, these observations are enough to indicate that something might be happening with vision and cannabis consumption. (Note: No one should drive at night or at any time under the influence of cannabis.) But why does this effect exist?

In a study recently published in the journal eLife, a group of researchers offer a potential answer.

To see what marijuana was doing to eye cells, Lois Miraucourt and co-authors decided to test the effect of cannabis to these cells on tadpoles of the African clawed toad (previous research has shown that the retinas of these frogs have cannabinoid receptors).

As Mo Costandi writes in The Guardian, the tadpoles "are transparent and, therefore, amenable to all sorts of experiments that cannot be performed in humans or other lab animals." That makes it easy to see what exactly is happening in the tadpoles.

Basically, the results showed that retinal ganglion cells, essential inner eye neurons, become more sensitive to light after being exposed to cannabis. The tadpoles themselves showed better ability to distinguish contrast after that exposure as well.

We still don't know exactly how useful medical cannabis will be for people. There are promising compounds, but so far, we haven't thoroughly studied exactly how they all work.

"Like many plants, marijuana has many different chemicals," Yasmin Hurd, a professor of neuroscience and psychiatry at Icahn School of Medicine at Mount Sinai, told me in an interview last fall. Hurd says using all the compounds in marijuana simultaneously is like "throwing 400 tablets in a cocktail and saying 'take this.'"